Geology Reference
In-Depth Information
the UK, it is normal to stop a test when 50 blows fail to advance the
split spoon the full 300mm and instead to record the penetration
achieved for the 50 blows. Depending on the ground conditions and
sample retrieved, it might be valid to extrapolate the blow count to an
equivalent N-value pro rata. Overseas, it is common practice to con-
tinue the test for 200 blows or more in weathered rock, and designs are
often based almost solely on such data, which is rather questionable
practice in a pro
le that might comprise a heterogeneous mix of harder
and softer materials. Tests carried out in this way may damage equip-
ment and are tedious for the drilling contractor, who might well be
tempted to cut corners if no-one is supervising. The interpretation
of SPT testing in weak and weathered rock is discussed in more
detail in Chapter 5.
The vane test involves rotation of a cruciform steel tool at a slow rate
within the soil ( Figure 4.33). The test is especially suitable for soft clay
where SPTs are inappropriate because of the indeterminate nature of
pore pressure changes brought about by rapid loading. The vane test is
assumed to give a direct measure of undrained shear strength for the
shape sheared by the rotating tool but interpretation can be dif
cult,
especially in bedded soils.
The static cone penetrometer is a conical tool (like an SPT) that is
pushed rather than driven into the ground, usually from a heavy lorry
( Figures 4.34 and 4.35) . The end force on the cone tip, and drag on the
sides of the tool, are measured independently and can be interpreted in
terms of strength and deformability. Clay, being cohesive, grips the
side proportionally more than sand or gravel, so the ratio between end
resistance and the side friction can be used to interpret the type of soil
Figure 4.33 Field vane used for measuring strength of clay down borehole or sometimes pushed
from the ground surface. Once at test location, the vane is rotated to measure shear strength of the
cylinder of soil de
ned by the vane geometry. To the left is a sleeve used to protect the vane during
installation.
 
Search WWH ::




Custom Search